The use in photographic cameras of built-in light meter devices, such as photodiodes and associated control circuits, for metering the illumination both before and during actual exposure of the film is well known. Such automatic exposure meters in non single lens reflex camera usually employ an imaging path separate from the taking lens path. In the case of single lens reflex cameras having interchangeable lenses, gauging of the proper exposure for film becomes a problem if separate light paths are used since lenses do not all have the same transmissivity characteristic. As a consequence, it is preferable that exposure meters for cameras with interchangeable lenses employ the same light path as used to expose the film, that is, to measure the incoming light through the taking lens. Such meter arrangements are also advantageous because of the ability to determine accurately the amount of incoming light for proper exposure of the film during a flash picture and to quench the flash when proper exposure has been achieved.
The use of so-called off-the-film (OTF) light metering in cameras is well known. In OTF metering, the light from an object being photographed is typically reflected by the camera shutter curtain which has been printed with a patterned material to resemble the reflectivity of film. The exposure control circuit measures the light reflected from the curtain and then proceeds to determine the proper exposure period. Upon initiating the film exposure, the circuit measures the light reflecting from the surface of the film and employs the shutter curtain measurement to correct the film reflectivity variable. An example of such a technique is found in U.S. Pat. No. 4,295,720 which describes an improved circuit that measures the light from a plain black curtain. A modification of this approach is described in U.S. Pat. No. 4,685,786 wherein one of two film reflection coefficients is selected by inferring whether the film is for slides or prints based on film latitude information DX encoded on the film cartridge. While useful as attempts to provide OTF metering, such techniques are significantly limited in accuracy by the variability that exists in the reflectance and scattering properties of different films. The film surface reflection is more variable than these prior art techniques can compensate.
As is known in the photographic art, the process of film manufacturing involves making emulsion and sensitizing them in batches. In the case of color film, the emulsion is sensitized to obtain the desired spectral sensitivity for each color absorption layer. After an emulsion batch is sensitized, a small coating sample is made for film characterization. Each batch is analyzed and then appropriate sensitivity adjustment coatings are specified to keep the speed of the respective color layers in balance. For example, if the green layer is a bit slow, the red and blue layers are filtered to reduce sensitivity so that the film maintains proper overall color balance. The various thicknesses of the resulting coatings interact with the impinging light (by wavelength) and affect the surface reflectance of the film, batch to batch. This variation occurs in addition to an already large film surface reflectance variability across film types. It is desirable, therefore, that calibration of exposure with OTF exposure control at the time of picture-taking be based on actual film characteristics and that exposure based on shutter curtain readings or inferred film characteristics be avoided.
It is also known in the photographic art that the rated ISO of a film corresponds to the exposure yielding first acceptable images. A common practice among experienced photographers is to adjust the ISO so as to further optimize film characteristics. For example, consumer negative film colors can become more saturated when the film is exposed at an ISO slower than the rated ISO. Similarly, slide film colors are more saturated when the film is exposed at an ISO faster than the rated ISO. The amount and direction of exposure bias selected is determined by the photographer based on testing a roll from his block of film, combined with his prior experience. Inexperienced or low volume film users do not benefit from this capability. If an exposure aim for optimal image quality was determined at film manufacturing and provided for all photographers, it would save experienced photographers time spent characterizing each batch and would also allow camera having intelligent exposure control to selectively utilize the aim to provide inexperienced users with higher quality images.